1. Field of the Invention
The present invention relates to a semiconductor device and to a method of forming a semiconductor device.
The present invention is concerned with a broad class of semiconductor devices, and particularly integrated circuits, including microprocessors, analogue and digital CMOS, BiCMOS and smart-power circuits. The present invention has particular applicability to devices that use silicon-on-insulator (SOI) technologies though it is applicable to other technologies such as the bulk planar technologies.
2. Description of Related Art
A known problem of conventional integrated circuits is that they can suffer from self-heating due to excessive power developed within the active region of the circuit which is converted to heat. Heat in such circuits may be developed during on-state or transient operation and can be caused by DC, AC or RF electrical power. This heat can cause high temperature effects such as latch-up, parasitic bipolar conduction, reduction of channel mobility, and threshold voltage variation in MOS devices, and generally can cause reliability problems. Local heating or hot spots are particularly damaging to circuit performance and reliability.
In order to dissipate the heat and therefore to prevent high temperatures from being developed in the integrated circuits, various techniques are known. Examples are disclosed in U.S. Pat. No. 5,895,972, U.S. Pat. No. 5,650,639, U.S. Pat. No. 5,354,717, U.S. Pat. No. 5,109,268, EP-A-0552475, EP-A-0317124 and WO-A-94/15359. In most of these prior art disclosures, a silicon substrate is deposited as a layer on or bonded as a wafer to a diamond substrate and then appropriate processing steps (including doping, etc.) are carried out on the silicon in order to form the semiconductor devices.
In U.S. Pat. No. 5,895,972 there is disclosed a method and apparatus for cooling a semiconductor device during the testing and debugging phases during development of a device. In place of conventional heat slugs such as copper, a heat slug of material that is transparent to infra red is fixed to the device. A diamond heat slug is disclosed as preferred. It is disclosed that the substrate on which the device is formed can be thinned prior to applying the infra red transparent heat slug to the device. The purpose of this thinning of the substrate is solely to reduce transmission losses that occur during optical testing and debugging of the device using infra red beams. This process is carried out during development of the device. The heat slug is not used during normal operation of the device. Indeed, the device that has been tested in this way will normally be destroyed by this process as this testing is carried out to obtain data on the class of devices as a whole and is not for example part of the normal testing of a device intended for retail.
U.S. Pat. No. 3,689,992 discloses a technique for isolating various regions of a semiconductor device from each other. In one of the examples described, various insulator layers are laid over the back of the device and then a support layer, specifically exemplified by polycrystalline semiconductor material, is laid over the back of these insulator layers.